The invention relates to an extrusion head for producing a tubular parison for the manufacture of blow-molded plastic hollow bodies, including an adjustable ring-shaped tube outlet nozzle with annular nozzle/mandrel gap adjustment elements which permit a targeted adjustment of the nozzle gap for varying the wall thickness of the exiting parison.
Typical examples of such blow-molded parts of thermoplastics, in which customarily a targeted wall thickness adjustment is carried out in longitudinal direction of the tube and in circumferential direction of the tube, include large-volume containers such as, for example, canisters, lidded drums, bung drums or inner receptacles for pallet containers. During ejection of the tubular parison used for such blow-molded parts, the wall thickness of the tube is so adjusted by a rotation-symmetrical nozzle/mandrel gap control mechanism in longitudinal direction of the tube that the wall is slightly thinner toward the lower end and slightly thicker toward the upper end in direction of the discharge valve, because the tube as a consequence of its own weight lengthens by itself when suspended to thereby automatically realize an even axial wall thickness. Superimposed thereto is a partial wall thickness control in circumferential direction which is realized by a double nozzle/mandrel gap control mechanism in the tube in those tube zones which are provided in the blow-molded hollow body for the head and bottom areas at greatly varying degrees of elongationxe2x80x94particularly in the areas which are offset by 90xc2x0 to the partition plane of the blow mold halves.
With double nozzle/mandrel gap control mechanisms (=two control choices) known heretofore, the wall thickness of such blow molded parts can be controlled over their length with respect to:
uniform radial wall thickness
radial and circumferential wall thickness
radial wall thickness with axially extending thick and thin areas, and
partial longitudinal thick/thin areas.
Various constructions of extrusion heads are known for such tube adjustments by means of nozzle/mandrel gap control, for example from German Pat. No. 26 54 001, U.S. Pat. No. 3,114,932 or U.S. Pat. No. 1,107,628. Although for standard applications, simple conventional or the double nozzle control mechanisms suffice, they lack the capacity to carry out additional influences of the wall thickness. A simple extrusion head is further disclosed in British patent specification 1,107,628 for producing teeth or ribs in the extruded tube for bottles blow-molded from thermoplastic material. By means of V-shaped grooves and teeth formed in the ejector die, the blown bottles should receive in particular a decorative look. The central mandrel can be adjusted axially so that the ejected tube may or may not be acted upon by the teeth. A further adjusting option or possible influence to permit a targeted wall thickness adjustment is, however, not possible.
It is an object of the present invention to obviate the mentioned drawbacks of the prior art and to improve the extrusion head of the type as described above with a double nozzle/mandrel gap control so as to permit a further additional superimposed wall thickness adjustment of the ejected tube for large-volume plastic containers such as 220 liters drums andxe2x80x94particularly also for asymmetric technical parts such as e.g. plastic fuel tanks (KKB) or for special applications in large cylindrical blow molded parts.
This object is attained in accordance with the invention by providing at least three separate nozzle/mandrel gap adjustment elements (D 0=mandrel, DS I, DS II, DS III) which are separately controlled, differently profiled, and adjustable, and which individually or/and simultaneously can be brought into active engagement in the nozzle gap with the exiting parison from inside and outside.
Since the tube outlet nozzle is equipped with a triple nozzle/mandrel gap control mechanism, special thick areas can be so adjusted at particular locations of the tube depending on the blow molded hollow body in dependence on the configuration of the third nozzle/mandrel gap adjustment element, or further adjustment elements and on the duration of the intervention or impact on the extruded parison, that the final blow molded hollow body exhibits only in the desired wall regions different or thicker wall thickness in comparison to the minimum wall strength of the typical wall.
A canister can, for example, be provided with stiffening ribs only at the vertical corner areas. In a KKB, the walls can also be made specifically thicker and thus more stable, e.g. in the side areas of the top bottom or at those areas at which connection stubs of fuel pipes are blow-molded thereon, or measuring instruments for a fuel tank level gauge are to be installed. In bung drums or lidded drums, it is possible to adjust a ribbed wall construction with uniform thick-thin distribution only in the vertical wall areas through a targeted material redistribution while, for example, maintaining the existing drum weight, thereby measurably improving the stiffness of the container, and in particular, the stacking strength when charged with hot contents. This is especially of particular importance in connection with large-volume drums (220 liters). Remarkably, the steps for quality improvement of the blow-molded hollow plastic body are realized exclusively on the exiting parison itself and not through constructive redesign of the blow mold.
As shown by way of example only, the novel triple or multi-nozzle/mandrel gap adjustment elements according to the novel invention results advantageously in a variety of new possibilities of application for large-volume blow-molded plastic parts of all types (e.g. automobile accessories or the like).
Compared to a conventional extrusion head with heretofore dual nozzle/mandrel gap adjustment elements, the provision of an extrusion head with the novel triple nozzle/mandrel gap adjustment elements according to the invention can be realized comparably inexpensively and does not require any investment in machines. The adjustment elements are moved only over very short distances, and only simple control motors are sufficient, and no deformation of the adjustment elements is experienced.
The process according to the invention for producing blow-molded hollow plastic bodies by a blow mold machine with extruder, extrusion head with circumferential distributor and respective blow mold, and carrying out a particular adjustment of the wall thickness of the exiting parison during ejection of the tubular parison from the extrusion head through adjustment of the nozzle/mandrel gap, is characterized in particular by realizing a random, circumferentially varying thick-thin adjustment of the tubular parison through a sequential or simultaneous impact of three differently profiled, separately adjustable nozzle/mandrel gap adjustment elements. This multiple adjusting capability of the tube for large-volume drums is unique and results in a quality of the drums that has been unattainable to date.